The invention relates to a method and a device for the reduction of the noise components in the data signal for an optical scanning device for the reading and/or writing of a magneto optic record carrier wherein a light source radiates light onto the record carrier and the light reflected from the record carrier is guided in dependence on its direction of polarisation onto a first photo detector or onto a second photo detector. Devices are envisaged, especially in connection with record carriers on which items of data are stored by means of pits and in magnetic domains, which make it possible to simultaneously read the items of data and to separate them from each other with the least possible mutual interference despite the superimposition on the record carrier. However, noise components already occur even with magneto optical discs in which the items of data are stored exclusively in magnetic domains since the magneto optical discs have different optical properties, for example different birefringence, so that the polarisation beam splitter which can only be optimally adjusted for one type of a magneto optic disc leads to interference components as a result of deviations from its optimal location.
For the reading of an optical or magneto optic record carrier, it is already known to extract the data signal representing the items of data stored in the magnetic domains of the record carrier from the difference of the photo electric voltages of the first and second photo detectors and to extract the data signal representing the items of data stored in the pits of the record carrier from the sum of the photo electric voltages of the first and second photo detectors c.f. DE-OS 37 32 874 A1. To this end, optical scanning devices are used which include a summing amplifier and a differential amplifier whereby the data signal representing the items of data stored in the pits of the record carrier at the output of the summing amplifier is supplied either via an amplitude and phase compensating network and a modulator to the laser driver or, in another embodiment, to a divider for the separation of the data signals. In a record carrier having both magnetic domains as well as pits, the intensity of the reflected light would constantly fluctuate and be superimposed as interference on the data signal. For the avoidance of this superimposition, the reflected light is, in a first embodiment, kept constant by means of a modulator. A data signal which represents the items of data stored in the magnetic domains is then derivable at the output of the differential amplifier. The items of data contained in the pits are comprised in the control signal of the modulator. Since the light output of the light source is modulated by the data signal coming from the pits, one also refers to this method as a laser feedback process. Therein however, the adjustment of the laser feedback amplification is particularly difficult since the laser feedback amplification has to be readjusted to the most suitable value for each record carrier or each pit of the record carrier. The laser feedback system has to be implemented as a wide band amplifier and thus tends to oscillate easily. The outlay necessary for the separation of the items of data stored in the magnetic domains and the pits of the record carrier is reduced in a second embodiment of an optical scanning device which utilises a divider for the separation of the data signals. The light source is not modulated with the data signal from the pits so that the modulator as well as the amplitude and phase compensating network are not required. In essence, the difference signal between the photo diodes is the data signal stored in the magnetic domains but which however, has superimposed thereon the items of data stored in the pits in the form of an amplitude modulation. Due to the use of a divider, it is so, that basically, the data signal stored in the pits is separated from the data signal stored in the magnetic domains but noise components occurring in particular in connection with the edges of the pits remain in the data signal of the magnetic domains.
Since, in contrast to the data signal stored in the pits, the data signal stored in the magnetic domains is alterable by a new recording, the data signal stored in the magnetic domains is also referred to as a RAM signal and the data signal stored in the pits is referred to as a ROM signal. The outlay is of course reduced due to the electronic separation of the ROM and RAM signals but, at the same time, the noise components coming for example from the pits are increased in the data signal of the magnetic domains.
For the reduction of the noise components in the data signal, it is known to regulate the intensity of the light reflected from the record carrier or transmitted through the record carrier at a constant value independently of the optical properties of the record carrier c.f. EP PS 310 812 B1. The reduction of the noise components is achieved exclusively by way of the laser feedback which demands a heavy outlay.
Therefore, it is the object of the invention to reduce noise components in the data signal without laser feedback and also at low cost.